The invention relates to a method for applying a protective coating to a substrate, in particular a metallic pipe and polymeric substrate.
To protect substrates such as metallic pipes and pipelines from, for example, corrosion or mechanical abuse, a protective coating may be applied.
Various coatings for such substrates are known and many are commercially available. One class of coatings that has been found to be effective include the so-called fusion bonded epoxy coatings. These coatings are applied to the substrate in powdered form and are then heated to relatively high temperatures, about 200.degree.-300.degree. C., to cause the powder particles to fuse and flow together and cure to form a continuous coating bonded to the substrate. Typically, the substrate is preheated to these high temperatures before the powdered resin is applied. For good adhesion between the epoxy coating and the pipe, it is generally required that the surface of the pipe be adequately cleaned and abraded. The fusion bonded coating may be further protected by application of one or more additional layers. Coating systems of this type are disclosed in U.S. Pat. Nos. 4,213,486 to Samour and 4,510,007 to Stucke. A modification of this coating approach is disclosed in U.S. Pat. No. RE30,006 to Sakayori et al in which a modified polyolefin is applied to an uncured epoxy resin coated on a metal surface and then the system is heated to melt bond the uncured epoxy resin and the polyolefin and to cure the epoxy resin. The temperature utilized must be above the melting point of the polyolefin, generally between 80.degree. and 350.degree. C.
Another coating technique is disclosed in U.S. Pat. No. 3,231,443 to McNulty in which a cured resin layer (e.g. epoxy) carried on a film backing is applied over an uncured resin layer (e.g. epoxy) coated on the substrate to be protected. The cured and uncured resins interact to form a protective coating. It is said that the coating can be applied in the field. Cured epoxy resins are generally brittle and thus precuring the resin as suggested in McNulty is likely to be difficult under actual pipe installation conditions, particularly if the pipe is of relatively small diameter.
Yet another approach to coating metal substrates is described in U.S. Pat. No. 3,502,492 to Spiller. In this approach a thin layer of powdered epoxy resin is coated on the metal surface and then a thick layer of plasticized polyvinyl chloride particles are applied. The coating is heated causing both the epoxy powder and the polyvinyl chloride powder to adhere to each other and to the underlying substrate. A continuous epoxy-polyvinyl chloride interface is not formed.
In general the above techniques because of the high temperatures and complex equipment required are limited to factory application of the coating or are otherwise impractical under actual field conditions. Application of pipeline coatings in the field, or protection of other substrates such as cable splices, require techniques that employ more moderate temperatures and portable equipment. One successful technique for coating pipelines in the field is the use of a heat-recoverable (i.e. heat shrinkable) polymeric article, such as a tube, sheet or tape, in combination with a heat activatable sealant. In installing such coatings, a torch or other heat source is generally used to heat the article to cause it to recover. This heat also raises the temperature of the heat activatable sealant to above its activation temperature so that a strong bond is formed between the polymeric article and the substrate. Suitable sealants include hot melt adhesives and mastics. There are certain applications in which the substrate may act as a heat sink and prevent the interface between the adhesive and the pipe from attaining the minimum bond line temperature required to form a strong bond between the adhesive and the pipe. One such situation is an oil-carrying pipeline in which the oil is between 25.degree.-50.degree. C. Under these conditions it is difficult to retrofit or repair a pipeline coating with a heat recoverable article such as described above.
We have now discovered a coating method which overcomes the deficiencies of these prior techniques. The methods of this invention do not require the high temperature and complex equipment requirements of fusion bonded epoxies and can be used to retrofit or repair existing pipelines carrying oil at temperatures below the activation temperature of typical heat activatable sealants. The method is versatile and can be used in a factory, at a field plant or "over the ditch" to apply protective coatings to various substrates in particular metallic pipelines. The method can be used to protect a variety of substrates including cables, cable splices, conduits, and the like.